In an automatic combination weighing machine, each weigh hopper is preferably loaded with a specific weight of product which is previously selected in accordance with a requested target combination combined weight. For the purpose of feeding product to such a weigh hopper, a retaining hopper is generally disposed just above the weigh hopper for retaining some amount of product and discharging its content into the weigh hopper by controlling its exit gate door. The exit gate is generally opened in response to unloading of the weigh hopper and closed in response to detection of a predetermined weight by the weigher. In order to obtain high speed operation, it is desirable to load the weigh hopper as fast as possible. However, if the product is discharged from the retaining hopper at a very high flow rate, a large overshoot of weight value is detected by the weigher and accurate loading is unobtainable.
In order to overcome this overshoot problem so-called triple step loading is widely used at present. In this method, the exit gate aperture is changed three times as shown by G1, G1' and G2 in FIG. 1 corresponding respectively to "rough", "medium" and "fine" feedings. As shown in the drawing, the gate aperture is changed abruptly at times t0, t1, t2 and t3 but, due to the vertical distance between the weigh and retaining hoppers, the response of the weigher is delayed respectively by times L0, L1, L2 and L3 as shown in FIG. 1(b). Also, in this method substantial overshoots as shown by A', B' and C' take place and result in substantial variance in the weight of product in the stabilized weigh hopper with respect to a predetermined target weight Wp. Therefore, it is difficult to determine the difference d between the target weight Wp and weight W3 corresponding to the time t3 at which the retaining hopper gate is to be closed.
Japanese patent opening gazette No. 60-238723 discloses a weigher proposed for overcoming the abovementioned overshoot problem. In the disclosed weigher, the gate aperture Gx between the times t1 and t2 is controlled linearly with respect to the detected weight Wx in accordance with the following equation. ##EQU1## FIGS. 2(a) and (b) show the relation between the gate aperture and the detected weight of product fed into this weigher. As seen from FIG. 2(b), the overshoot phenomena A',C' and resultant variance in the final weight still remain.